1. Field of the Invention
The present invention relates to a stencil printing machine such that a squeegee roller with ink provided on an outer circumferential surface thereof contacts an inner circumferential surface of a printing drum, and that the ink is forced to pass through perforations of a stencil sheet to transfer onto a printing sheet, thereby conducting printing. More specifically, the invention relates to an improved mechanism for adjusting a gap between the squeegee roller and a doctor roller that is movable relative to the squeegee roller.
2. Description of the Related Art
A stencil printing machine has a printing drum having a stencil sheet wrapped thereon and a squeegee roller situated inside the printing drum. As illustrated in FIG. 14, an outer circumferential surface of a squeegee roller 1 contacts an inner circumferential surface of a printing drum 3. And, in the printing drum 3, an ink-control device, i.e. a doctor roller 5, is situated parallel to the squeegee roller 1 at a predetermined gap from the outer circumferential surface of the squeegee roller 1. This constitution allows ink to make a wedge-form reservoir 7 (ink reservoir) between the outer circumferential surfaces of the squeegee roller 1 and the doctor roller 5.
In the ink reservoir, an ink mixing shaft 9 is rotatably situated parallel to the squeegee roller 1. The ink mixing shaft 9 has a gear 11 attached to one end thereof. The gear 11 engages with a gear 13 fixed to one end of the squeegee roller 1, thereby being rotated in a direction opposite to that of the squeegee roller 1 in synchronization with rotation of the squeegee roller 1.
During operation of the stencil printing machine, the printing drum 3 rotates in a direction "A" shown in FIG. 14, and the squeegee roller 1 rotates in a direction "B" shown in the same. This movement urges an ink of the ink reservoir 7 to swirl around the ink mixing shaft 9 in a direction "C", thereby forming an ink lump there. The ink forming the lump is supplied by an ink supplying device comprising an ink pump, an ink conducting pipe, and an ink distributing pipe (distributor). These components are not shown in the drawing.
In the stencil printing machine, control of ink-quantity supplied to the squeegee roller 1 is important for ensuring printing quality. The ink-quantity is controlled by a gap between the squeegee roller 1 and the doctor roller 5. Therefore, the doctor roller 5 is supported by a mechanism allowing the roller to move relative to the squeegee roller 1 to adjust the gap.
The mechanism is approximately composed of a bearing member 15 of the squeegee roller 1, an adjusting screw 17 with one end portion screwed in the bearing member 15, an end portion 19 of an axis of the doctor roller 5, and a compression coil spring 21. The adjusting screw 17 passes through a hole formed in the end portion 19, and the compression coil spring 21 is fitted on the adjusting screw between the bearing member 15 and the end portion 19. The other end portion of the adjusting screw 17 is an enlarged head 17a. The head 17a contacts and holds the end portion 19 of the doctor roller 5, thereby preventing the portion from coming off the adjusting screw 17. In other words, the end portion 19 of the doctor roller 5 is always urged towards the head 17a by the compression coil spring 21.
In this state, there is formed a gap in a predetermined width between the squeegee roller 1 and the doctor roller 5. In order to make the gap narrower, the adjusting screw 17 is turned in the screwing direction by a tool such as a screwdriver that is engaged with the head 17a on each end of the doctor roller. Then, the end portion 19 of the doctor roller 5 moves towards the bearing member 15 while resisting the urging force of the compression coil spring 21. As a result, the gap becomes narrower. Contrary to this, if the gap is required to become broader, the adjusting screw 17 is turned in the unscrewing direction. Then, the end portion 19 of the doctor roller 5 is moved in a direction departing from the bearing member 15 by urging force of the compression coil spring 21. As a result, the gap becomes broader.
Thus, in the conventional stencil printing machine, the adjusting screws 17 on both end portions of the doctor roller are turned by the tool, thereby moving the doctor roller 5 relative to the squeegee roller 1 to adjust the gap. This operation controls ink-quantity supplied to the squeegee roller, thereby achieving printing of high quality.
In the stencil printing machine, the gap between the squeegee roller and the doctor roller changes according to abrasion of the squeegee roller. However, since adjusting of the gap according to abrasion of the squeegee roller is not frequently required, the gap-adjusting mechanism explained above can facilitate adjusting of the gap due to such abrasion.
However, aluminum or rubber as surface material of the squeegee roller expands and contracts according to environmental temperature. For example, some kind of material among them allows the gap to change by 0.004 mm/.degree. C. On the other hand, an allowable limit to fluctuation of the gap is 0.04 mm. Accordingly, when temperature fluctuates over 10.degree. C., fluctuation of the gap exceeds the allowable limit. This makes printing density in the stencil printing unstable. Therefore, adjusting of the gap according to temperature fluctuation is frequently required.
However, in the conventional gap adjusting mechanism, the gap must be adjusted approximately by 1/100 mm by turning an adjusting screw having pitch of approximately 0.7 mm. Accordingly, the adjusting screw is required to turn in a subtle angle within one rotation, and skilled technique ensured by enough experience and high-sensitivity of an operator is required to conduct such adjusting operation.
Further, in the conventional gap adjusting mechanism, both ends of the doctor roller are adjusted by independent two adjusting screws, respectively. Such adjusting operation is troublesome, and parallelism between the rollers is difficult to be achieved.
Further, in a stencil printing machine such that the squeegee roller moves vertically in the printing drum, the stencil sheet must be removed from the printing drum when the adjusting screw is operated to turn for the gap-adjusting. Namely, the gap-adjusting is accompanied by such additional operation; therefore, it is further troublesome.
More further, in the conventional constitution having the independent two adjusting screws, operation of the gap-adjusting is subtle. And, such constitution is so complicated that it hinders an automatic gap-adjustment system with a motor as a driving source from being realized.
The present invention is made in view of the forgoing. An object of the present invention is to provide a stencil printing machine in which the gap-adjusting can be easily conducted without skilled technique of an operator.